Sonar transducers are often carried by ships to serve as depth sounders and to perform other functions such as the locating of fish and the measurement of the ship's speed. The transducer of the sonar system is positioned within a housing having a sound transmissive window located on the keel of the ship, the housing being preferably located near the bow of the ship in order to reduce the amount of noise received from the ship's screws.
Generally, ships, such as war ships, passenger cruise ships and oil tankers, sail with the water line at the surface of the water so that the transducer housing is well below the water surface and thereby protected from the forces of the waves. Except in the case of very stormy seas, it is unlikely that the window of the housing will be lifted out of the water.
However, a problem has arisen in the case of certain freighters, such as those used in the ferrying of automobiles, in that the freighters may carry cargo in only one direction and return empty, riding high in the water. The draft of the empty freighter is so small that the bow of the freighter may lift out of the water during normal seas with the result that the window may often crash hard against the surface of a wave. When the window is constructed of a rigid plate of sound transmissive material such as acrylonitrile-butadiene styrene (ABS plastic), the window has been found to fracture with the unfortunate loss of the function of the sonar system. Windows have also been constructed of flat plates of elastomeric material which are secured to a mounting ring, the material being in a state of tension resulting from shrinkage during the curing of the material. In such windows, the material tends to slowly weaken at the interface between the plate and the ring and, eventually, separates from the ring. Thereupon, the waves tear the plate from the ring admitting the ocean water into the housing with the loss of the sonar function.